This invention relates to an apparatus and process for tinting plastic material and more specifically, for tinting polycarbonate and other plastic eyeglass lenses
Presently, eyeglass lenses are made from either glass or plastic. Plastic lenses have advantages over glass in that they are lighter and can be easily tinted by an optical laboratory using procedures well known in the art.
The advent of the development of clear plastic materials such a plexiglass offered the manufacture and use of light weight eyeglass lenses. The main disadvantage was that of scratching of the surface of the lens rendering them opaque.
PPG has marketed CR-39 (diethylene glycol bis(allyl carbonate) monomer for over a decade for the manufacture of plastic eyeglass lenses. CR-39 exhibits the best abrasion resistance and hardness of all plastics considered for use in eyeglass lenses. The impact resistance is carefully controlled to pass FDA impact specifications. Improvements have been made in commercial lenses with the addition of hard abrasion resistant coatings. Silicone (silica/silane) heat cured coatings and acrylic ester ultraviolet light cured coatings are used. Hard coats are preferred on the front surface of the lens but are often not used on the back surface of a prescription lens. The CR-39 surface also tints with relative ease using disperse dyes prepared especially for this use at moderate temperatures. The extremely hard coatings however have much less affinity for tint.
Polycarbonate (a thermoplastic) supplied by General Electric (Lexan TM) and others has the unique advantage of high refractive index which allows for the use of thinner lenses, lighter lenses. Optical lenses made from this material are more comfortable in that there is less strain due to the weight of the lenses on a wearer's nose and ears. It is also much more resistant to impact and is truly a shatterproof safety lens. It is however so soft that it cannot be used without a hard coating on both the front and back surfaces. An additional reason for coating polycarbonate lenses is that the polycarbonate surface will not absorb the same tints that are used commercially for the higher volume CR-39 lenses.
More recently many other high refractive index thermoplastic and thermoset lens have appeared in the marketplace. Although many are currently recommended for use without hard coatings most are softer than uncoated CR-39 and additional use will dictate the use of hard coatings.
The lens manufacturer generally produces plastic lens blanks with a finished front surface as described above. An optical laboratory then shapes, fines and polishes the prescription into the rear surface. In the case of the softer lenses a hard coating is then applied. The coating selected may be very hard and abrasion resistant if a clear lens or only a fashion tint is desired. In general a less abrasion resistant specially formulated material is required as a coating if a sunglass depth of tint is desired. The lens is generally edged after coating and then tinted to the desired color and depth of tint. The tinting operation is the most time consuming operation in an optical laboratory.
Heatable tanks used in the plastics dyeing process are well known. A conventional heatable dyeing tank includes heating elements positioned in the bottom of an outer tank. Such elements are in contact with a heat transfer media, into which the dyeing tanks are placed. The tank is heated by conduction of the heat transferred from the heating elements to the tank via a heat transfer media such as silicone or polyethylene glycol, and then to the outer surfaces of the tanks.
To use the tank, a holder containing lens material to be dyed is positioned into the dyeing tank. The commonly used methods and apparatus for dyeing lens material use a dyeing tank whose heater output can be set at a variety of temperatures.
The general directions for using tints recommend using a temperature range of 190 to 205 deg. F. The available apparatus however will not control to such tight tolerances. Current tints boil at approximately the same temperature as water 212 deg. F. Covers are supplied with the apparatus to slow the evaporation of water when tinting is not being performed.
Because of the nature of the heating mechanism of current equipment the operator must choose whether to use the covers or not. If the operator chooses to use covers then the heat transfer fluid must be close to 212 deg. F. to prevent boil over. When the covers are removed to allow placement of a lens in the bath the temperature will drop quickly (20 minutes) to about 170 to 175 deg. F. At this low a temperature tinting of the harder lenses and coatings essentially ceases. If one operates the heat transfer fluid at about 238 deg. F., a bath temperature of 194 Deg. F. can be maintained with the covers off. Evaporation of water is severe. If the covers are placed on the tint tanks a boilover will occur in 7 minutes.
Additionally, the evaporative loss of water from the open dyeing tanks requires the manual addition of fill water to the dyeing tanks by the operator. Maintenance of an improper level of dye solution due to irregular or infrequent water replenishment by the operator may also affect the bath temperature, time and efficiency of the lens dyeing process.
Under current methodology, CR-39 plastic lenses are tinted by the following procedure. After finishing, the lenses are dipped in a dye solution. The duration of time the lens is submerged in the dye solution determines the degree to which the lens is tinted. If harder surfaced lenses are required, some optical laboratories coat the lens to provide greater scratch resistance. The coating is then the surface that absorbs the tint. All of the optical laboratories, as well as many retail outlets, are set up to perform the above-described tinting procedure.
Many conventional dye stuffs can be used for the purpose of this invention. Many "disperse dyes" and mixtures thereof have been found satisfactory. Disperse dyes are actually very fine particles of the chemical color. The color is then absorbed from these particles. Examples of such disperse dyes include Disperse Blue #3, Disperse Blue #14, Disperse Yellow #3, Disperse Red #13 and Disperse Red #17. The classification and designation of the dyes recited in this specification are based on "The Color Index," 3rd edition, published jointly by the Society of Dyes and Colors and the American Association of Textile Chemists and Colorists (1971). Chemical identities of the above-mentioned dye stuffs can be found in that publication. Dye stuffs can generally be used either as a sole dye constituent or as a component of a dye mixture depending upon the color desired.
A typical dye concentration in the bath is 2% by weight, but there is a considerable latitude in this regard. Generally, dyes may be present in the water dispersion at a level of about 0.2 to 15%, preferably 1 to 4%, based on the total weight of dye solution. Where a dye mixture is used and the rates of consumption of the individual dyes are different, dye components will have to be added by immersing the lens in the color that has been depleted. The tints are normally discarded after about a week of use.
Additionally, commercially available dyes are presently mixed with distilled water. These dye dispersions, therefore, have a boiling point of 212.degree. F. (or 100.degree. C.). As is well known in the art, a 10.degree. C. increase in dye temperature at this level produces a substantial increase in dye affinity to the lens material, but the present limiting factor is the boiling point of the solution comprised of distilled water, combined with the presently available commercial plastic dyes.
The conventional dyes are formulated to initially disperse evenly throughout a dyeing solution, but as the dye solution sits within the dyeing tank, the dispersion of dye pigment typically begins to settle out and collect on the bottom of the dyeing tank. This settling out of the dye particles, in conjunction with the elevated temperature of the bottom of the dyeing tank, produces a coagulation or charring of the dye particles, thereby reducing the effectiveness of the dye and requiring early replacement of the dye or tint.